1. Field of the Invention
The present invention relates to image detectors the type in which a matrix of photosensitive elements is associated with an additional light source. The invention can be applied in an especially promising manner to the detection of radiological images.
At present, it is usual to find large-sized image detector panels using matrices of photosensitive dots made of semiconductor materials, manufactured in particular by the technique of thin-film deposition. Most frequently, the photosensitive dots consist of amorphous silicon. Each of them comprises a photosensitive element (a photodiode for example) associated with a switch element constituted by a transistor or by a so-called switching diode.
The photosensitive dots form an array of rows and columns. The line-by-line activation of each of the switching elements enables the transfer, to a column, of the charges produced by the corresponding photodiode during the exposure of this photodiode to a measurement light signal, namely to a signal corresponding to an image to be detected. External multiplexer circuits are then used to read the charges of the different photosensitive dots.
2. Description of the Prior Art
A photosensitive matrix of the type indicated here above as well as its mode of operation and a way of making it are described in particular in the French patent application No. 86.14.048 (publication No. 2.605.166).
For the detection of radiological images, it is enough to interpose a scintillator between the photosensitive matrix and the X-rays in order to convert the X-rays into visible radiation or near-visible radiation to which the photodiodes are sensitive. It is the conventional practice, to this end, to coat the photosensitive elements of the matrix with a layer of a scintillating substance such as thallium-doped cesium iodide.
There are different methods of controlling the operation of the photosensitive matrices. These methods depend especially on the particular goals that are aimed at, the general organization of the matrix as well as the constitution of the photosensitive dots.
Some of these methods of control use an additional illumination of the photosensitive dots by means of an additional light source. This additional light source produces an additional light beam (hereinafter called an additional radiation) that is independent of the measurement light signal corresponding to an image to be detected. This case is fairly frequent, especially when the photosensitive dots are each constituted by a photodiode that is series-connected with a switching diode. Indeed, in this case, the exposure of the photosensitive dots to additional radiation is a relatively simple means of taking action on the value of a voltage present at the junction point of the photodiode and a switching diode, in each of the photosensitive dots.
This exposure of the photosensitive dots to additional radiation may constitute what is called an optical resetting or optical level-resetting operation. It is generally performed in a stage of the method that precedes the stage of exposure to the useful measurement signal. An optical resetting operation of this kind is described in the patent application No. 86.14.058 already mentioned here above.
Other methods of operation are known wherein an additional illumination of the photosensitive matrix is accomplished in order to create so-called drive charges in each of the photosensitive dots. A method of operation of this kind is described in the French patent application No. 88.12126 published under No. 2.636.800. In this operation, for each photosensitive dot, driving charges are added to the "signal" charges produced by the measurement light signal in order to improve the efficiency of transfer of these charges for the low values of the signal.
Whatever the reasons for exposing the photosensitive matrix to additional radiation, the additional light source (hereinafter called a "additional source") which produces this radiation is generally positioned on a rear face of the support of the photosensitive matrix, namely on a face opposite the one on which the matrix is made. An arrangement of this kind is made possible through the transparency of the support which is generally made of glass. Consequently, the level of transparency of the support is of great importance because the higher this level of transparency the greater is the degree to which, for the same result, it enables the emission of the additional radiation with lower power and hence provides for the greater compactness and lower cost of the additional radiation source.
However, this transparency of the support which is useful for additional illumination may also have a major drawback which is explained in greater detail with reference to FIG. 1.
FIG. 1 gives a schematic view of a cross-section (parallel to a line of photosensitive dots for example) of the structure of a standard image detector panel 1 of the type defined here above. The image detector has a matrix 2 of photosensitive dots p1, p2, p3 each formed in the example by a switching diode Dc associated with a photodiode Dp. The photosensitive matrix 2 is made on a first face 3 of a transparent support or substrate 4, made of glass for example. The image detector 1 furthermore has an additional source 6 applied to the second face or rear face 7 of the support 4 opposite the photosensitive matrix 2. The additional source 6 (using for example photoemissive diodes that are not shown) produces a radiation constituting the additional radiation mentioned here above, designed for example to obtain an optical resetting operation.
The photosensitive matrix 2 is illuminated by a so-called incident radiation represented by rays Ri. This radiation is the measurement light signal. The incident light to be detected is generally not entirely absorbed by the photosensitive dots p1, p2, p3. Indeed, a small part of the incident radiation Ri goes through the active material of these photosensitive dots (the amorphous silicon of the photodiodes Dp) without being absorbed. Another part goes through the detector panel 1 between the photodiodes in the free spaces between connections (not shown).
After the crossing of the glass substrate 4, as indicated in FIG. 1 by the paths tr1, tr2, . . . , trn, this fraction of incident light which has not been absorbed and therefore not detected is reflected or scattered uncontrollably. This light fraction then constitutes a parasitic light which may finally be detected by photosensitive dots other than the dots close to which it has passed or in which it has passed and more particularly through dots which are distant from it. The additional source 6 is shown as being applied against the substrate 4. It is possible to envisage an embodiment in which the additional source 6 and the substrate 4 are separated by a free space. Its thickness may be one or more millimeters or even one or more centimeters. This space forms a substrate-air diopter with the substrate. Light can be propagated in this space.
The overall result is a loss of contrast that is particularly pronounced at the low spatial frequencies.
One solution aimed at reducing the level of this parasitic light, drawing inspiration from techniques used in the field of image intensifiers, consists of the use of a less transparent support having a transmission T. In this case, the undetected light must go twice through the support in order to be picked up by a photosensitive dot and it is therefore attenuated by a factor T.sup.2. It is thus possible to make the parasitic light negligible. This approach has the drawback by which, in particular, the additional radiation must be 1/T times more powerful in order to fulfil its function.
It is an aim of the present invention to reduce the loss of contrast mentioned here above without in any way thereby having to modify the emitted power of the additional radiation.